It has long been known that recoverable articles are useful for covering, protecting and/or encapsulating other articles. For example, elastomer sleeves have often been used to cover cylindrical articles merely by choosing a sleeve which, in its relaxed condition, has a diameter less than that of the article to be covered. The tendency of the elastic sleeve to retract when it has been expanded and placed over the article results in a covering for the article which, for some few purposes, is satisfactory. Heat recoverable encapsulating articles have been used in a somewhat similar manner and generally found superior to elastomer covers in many applications. However, many of the heat recoverable encapsulating articles known to the prior art have not been completely satisfactory particularly under circumstances where a strong and impervious bond between the recoverable article and the workpiece which is to be covered is desired. For example, it is common practice to laminate an insulating covering over electrical components such as conductors. The insulating material must, of course, be securely bonded to the conductor in order to protect the conductor from water or air or other media with which it may come into contact. Furthermore, it is also highly important that there be no air spaces between the convering and the conductor. The occurrence of air spaces is particularly troublesome where the insulating material is applied to an element having an irregular surface, such as a plurality of wires which have been twisted together to form a cable or braided structure. Thus, as is well-known to those skilled in the art, there has been a long standing need for heat recoverable coverings which are easy to apply, and yet are capable of forming a secure, intimate, impervious bond with the article which is covered.
U.S. Pat. Nos. 3,243,211 and 3,396,460 advanced the art when they taught a means of providing a heat recoverable article capable of being laminated to another article in such a manner that a secure, intimate and impervious bond was formed. Copending, commonly assigned application Ser. No. 129,919, filed Mar. 31, 1971, and now abandoned teaches the use of heat recoverable sleeves to repair breaks in mine cables. The disclosures of said patents and said applications are incorporated herein by reference to avoid unnecessary enlargement of the instant specification by the inclusion of an extensive discussion of the known prior art. This prior art teaches, inter alia, a heat recoverable, hollow article provided with a fusible insert, i.e., liner. The terms "insert" or "liner" as used hereinafter contemplate any fusible member which is positioned such that it will be interposed between the recoverable encapsulating member and the article, i.e., substrate, which the recoverable member is to encapsulate. The term fusible liner, for the purposes of this application shall include both liners of material that have a definite fusing point and materials that simply become more flowable as their temperature is increased. The words "sleeve" or "jacket" will, for convenience sake, be used hereinafter to describe any hollow, heat-recoverable encapsulating member. The prior art has found that an essentially abutting relation of the liner with both the recoverable sleeve and the substrate article to which the recoverable sleeve is to be united as preferred. Under these circumstances a heat induced change, i.e., recovery, of the recoverable member will urge the finish member toward the substrate and bring it into compressive abutment therewith.
However, to achieve a secure, intimate and impervious bond between the sleeve and the substrate the interposed fusible member must become "fluid", i.e., be reduced in viscosity to an extent sufficient that it flows and "wets" the opposing surfaces of the substrate and of the recoverable member and thereby forms a bond therebetween. A material is said to flow or be flowable if it has substantially no stability of form under the pressure exerted on it by the recoverable member. The term fusible material includes materials showing a sharp drop in viscosity over a relatively narrow range of temperatures. However, it should be understood that materials which exhibit a gradual decrease in viscosity over a large temperature range may be used in the same manner as fusible materials in accordance with this invention. With polymeric substances the term "flow temperature" therefore connotes the temperature at which the polymer has a sufficiently low viscosity to wet surfaces it contacts. With substantially crystalline polymers this flow temperature approximately corresponds to the polymer melting point and this latter term shall be used hereinafter to connote the flow temperature of both crystalline and non-crystalline polymers. Thus to achieve a good bond sufficient heat should always be applied to the fusible insert, i.e., liner, to raise it above its melting point. Under these circumstances where the recoverable sleeve is thin and its heat recovery temperature substantially greater than the melting point of the fusible liner this is readily achieved. However, frequently when thick recoverable sleeves and/or comparatively high melting fusible inserts are used, an unskilled worker will not apply sufficient heat to fully melt the fusible liner. This is, of course, highly undesirable since the liner will then not fully wet the surfaces of both the substrate and the sleeve and a sufficiently secure, intimate and impervious bond may not be formed therebetween.